Chapter 19. Profile Mode

Chapter 19. Profile Mode
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Intro

+ Goal: Give performance improvement advice based on + recognition of suboptimal usage patterns of the standard library. +

+ Method: Wrap the standard library code. Insert + calls to an instrumentation library to record the internal state of + various components at interesting entry/exit points to/from the standard + library. Process trace, recognize suboptimal patterns, give advice. + For details, see + paper presented at + CGO 2009. +

+ Strengths: +

+ Unintrusive solution. The application code does not require any + modification. +
The advice is call context sensitive, thus capable of + identifying precisely interesting dynamic performance behavior. +
+ The overhead model is pay-per-view. When you turn off a diagnostic class + at compile time, its overhead disappears. +

+ Drawbacks: +

+ You must recompile the application code with custom options. +
You must run the application on representative input. + The advice is input dependent. +
+ The execution time will increase, in some cases by factors. +

Using the Profile Mode

+ This is the anticipated common workflow for program foo.cc: +

+$ cat foo.cc
+#include <vector>
+int main() {
+  vector<int> v;
+  for (int k = 0; k < 1024; ++k) v.insert(v.begin(), k);
+}
+
+$ g++ -D_GLIBCXX_PROFILE foo.cc
+$ ./a.out
+$ cat libstdcxx-profile.txt
+vector-to-list: improvement = 5: call stack = 0x804842c ...
+    : advice = change std::vector to std::list
+vector-size: improvement = 3: call stack = 0x804842c ...
+    : advice = change initial container size from 0 to 1024
+

+ Anatomy of a warning: +

+ Warning id. This is a short descriptive string for the class + that this warning belongs to. E.g., "vector-to-list". +
+ Estimated improvement. This is an approximation of the benefit expected + from implementing the change suggested by the warning. It is given on + a log10 scale. Negative values mean that the alternative would actually + do worse than the current choice. + In the example above, 5 comes from the fact that the overhead of + inserting at the beginning of a vector vs. a list is around 1024 * 1024 / 2, + which is around 10e5. The improvement from setting the initial size to + 1024 is in the range of 10e3, since the overhead of dynamic resizing is + linear in this case. +
+ Call stack. Currently, the addresses are printed without + symbol name or code location attribution. + Users are expected to postprocess the output using, for instance, addr2line. +
+ The warning message. For some warnings, this is static text, e.g., + "change vector to list". For other warnings, such as the one above, + the message contains numeric advice, e.g., the suggested initial size + of the vector. +

Three files are generated. libstdcxx-profile.txt + contains human readable advice. libstdcxx-profile.raw + contains implementation specific data about each diagnostic. + Their format is not documented. They are sufficient to generate + all the advice given in libstdcxx-profile.txt. The advantage + of keeping this raw format is that traces from multiple executions can + be aggregated simply by concatenating the raw traces. We intend to + offer an external utility program that can issue advice from a trace. + libstdcxx-profile.conf.out lists the actual diagnostic + parameters used. To alter parameters, edit this file and rename it to + libstdcxx-profile.conf. +

Advice is given regardless whether the transformation is valid. + For instance, we advise changing a map to an unordered_map even if the + application semantics require that data be ordered. + We believe such warnings can help users understand the performance + behavior of their application better, which can lead to changes + at a higher abstraction level. +

Tuning the Profile Mode

Compile time switches and environment variables (see also file + profiler.h). Unless specified otherwise, they can be set at compile time + using -D_<name> or by setting variable <name> + in the environment where the program is run, before starting execution. +

+ _GLIBCXX_PROFILE_NO_<diagnostic>: + disable specific diagnostics. + See section Diagnostics for possible values. + (Environment variables not supported.) +
+ _GLIBCXX_PROFILE_TRACE_PATH_ROOT: set an alternative root + path for the output files. +
_GLIBCXX_PROFILE_MAX_WARN_COUNT: set it to the maximum + number of warnings desired. The default value is 10.
+ _GLIBCXX_PROFILE_MAX_STACK_DEPTH: if set to 0, + the advice will + be collected and reported for the program as a whole, and not for each + call context. + This could also be used in continuous regression tests, where you + just need to know whether there is a regression or not. + The default value is 32. +
+ _GLIBCXX_PROFILE_MEM_PER_DIAGNOSTIC: + set a limit on how much memory to use for the accounting tables for each + diagnostic type. When this limit is reached, new events are ignored + until the memory usage decreases under the limit. Generally, this means + that newly created containers will not be instrumented until some + live containers are deleted. The default is 128 MB. +
+ _GLIBCXX_PROFILE_NO_THREADS: + Make the library not use threads. If thread local storage (TLS) is not + available, you will get a preprocessor error asking you to set + -D_GLIBCXX_PROFILE_NO_THREADS if your program is single-threaded. + Multithreaded execution without TLS is not supported. + (Environment variable not supported.) +
+ _GLIBCXX_HAVE_EXECINFO_H: + This name should be defined automatically at library configuration time. + If your library was configured without execinfo.h, but + you have it in your include path, you can define it explicitly. Without + it, advice is collected for the program as a whole, and not for each + call context. + (Environment variable not supported.) +

+ Perflint: A Context Sensitive Performance Advisor for C++ Programs + . Lixia Liu. Silvius Rus. Copyright © 2009 . + Proceedings of the 2009 International Symposium on Code Generation + and Optimization + .

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Chapter 19. Profile Mode

Intro

Using the Profile Mode

Tuning the Profile Mode

Bibliography